Switch to sbt's fixed sha1

Change from sbt/sbt#1509 broke building sbt with Scala 2.11
This problem is tracked in sbt/sbt#1523
For now, we stick to fixed sha1 of sbt right before the merge of
sbt/sbt#1509.
Merge pull request #43 from gkossakowski/sbt-fixed-sha

Switch to sbt's fixed sha1